Generally, electrophotographic image forming apparatuses such as copiers, printers, facsimile machines, or multifunction machines including those capabilities include a development device to develop latent images formed on a latent image bearer with developer. Two-component developer consisting essentially of toner particles and magnetic carrier particles is widely used in image forming apparatuses.
Two-component development devices typically include a development roller, a developer container for containing developer supplied to the development roller, and two developer agitators provided in the developer container to agitate and transport the developer. The development roller includes a rotary development sleeve (developer bearer) and a magnetic field generator provided inside the development sleeve.
The magnetic field generator generates a magnetic force for pumping up the developer from the developer container onto the surface of the development sleeve and a magnetic force for transporting the developer to a development range where the development sleeve faces the latent image bearer. For example, to facilitate conveyance of the developer, the surface of the development sleeve is sandblasted or bead-blasted so as to form grooves or irregularities in its surface. Abrading the surface of the development sleeve by forming grooves or sandblasting can prevent or reduce slippage of the developer on the surface of the development sleeve and accumulation of the developer thereon, thus preventing a decrease in image density resulting from it.
Development sleeves having grooves on its surface can transport developer more reliably than sandblasted development sleeves because the grooves are larger than the surface irregularities by sandblasting and the durability is higher. However, the amount of the developer carried on the development sleeve is greater in portions where grooves are formed, and the image density becomes higher in portions corresponding to the grooves. Thus, the pitch of grooves causes unevenness in the image density, degrading the image quality.
Therefore, U.S. Pat. No. 7,925,192-B2 proposes forming multiple oval recesses in the surface of the development sleeve. With the recesses, the density of developer carried on the development sleeve can be as fine as that carried on blasted sleeves, and simultaneously development sleeves having the recesses are less likely to wear over time.
In such development devices, developer is circulated in the developer container by two parallel developer agitators provided therein that transport the developer in the opposite direction. The developer is thus agitated to uniformize charge amount of toner carried on the development sleeve and the concentration of toner in the developer.
FIGS. 34A illustrates a configuration of a known development device.
The development device shown in FIG. 34A includes a development roller 200 to transport developer G to a development range facing an image bearer 201, developer containing compartments 204a and 204b, and developer agitators 205a and 205b to transport the developer G in opposite directions. The development roller 200 includes a cylindrical development sleeve 202 and a magnet roller 203 provided inside the development sleeve 202 to generate a magnetic field. The developer G is pumped up to the surface of the development sleeve 202, attracted by the magnetic force exerted by the magnet roller 203. After a doctor blade 206 adjusts the layer thickness of the developer G carried on the development sleeve 202, the developer G is transported to the development range, where toner in the developer G adheres to the electrostatic latent image on the image bearer 201, developing it into a toner image.
In response to demands for high-quality images and energy saving, small-diameter toner having a lower melting point is preferred. Small-diameter toner having a lower melting point is less durable against stress, and changes in the properties thereof can affect image development.
In the above-described development device in which the developer is circulated, properties of the developer can differ between an upstream portion and a downstream portion in the direction in which the developer is transported (hereinafter “developer conveyance direction”) since the developer in the downstream portion has received stress repeatedly from the doctor blade and in the development range. More specifically, the developer in the upstream portion in the developer conveyance direction has a higher developability because fresh toner is supplied thereto. By contrast, toner in the developer in the downstream portion in the developer conveyance direction is degraded by the stress from the doctor blade and in the development range and has a reduced developability. The difference in the developability in the longitudinal direction of the image formation range appears as unevenness in image density.
To address the above-described problem, unidirectional circulation methods in which developer once used is not reused, and improvement of conveyance ability of developer agitators are proposed. However, in unidirectional circulation type development devices, the velocity at which the developer is transported should be increased, and thus enhanced transport ability (i.e., rotational frequency) is required, resulting in increases in size of the development device as well as increases in the stress on the developer.
In addition, in axial end portions of the development roller, the developer that has been used in image development (toner concentration is lower) tends to fail to leave the development sleeve and is again supplied to the development range, which is described in further detail below.
The magnet roller 203 shown in FIG. 33A has multiple fixed magnetic poles, for example, a development pole S1, a developer conveyance pole N1, an upstream release pole S2, an attraction pole or downstream release pole S3, and a developer conveyance pole N2. As the development sleeve 202 rotates, the developer G carried on the development sleeve 202 passes by the magnetic poles S3, N2, S1, N1, and S2 sequentially, and the developer G is separated from the development sleeve 202 in a release portion upstream from the attraction pole S3 in the direction in which the development sleeve 202 rotates.
After passing through the development range, the developer G falls in the developer containing compartment 204a due to the repulsive force between the magnetic poles S2 and S3. When another south pole P is provided between the magnetic poles S2 and S3, the repulsive force of the magnetic pole P can contribute to the separation of developer from the development sleeve 202. The developer G having a reduced toner concentration is mixed with developer having a higher toner concentration, transported from the developer containing compartment 204b, in the developer containing compartment 204a. Subsequently, the mixed developer is pumped up to the development sleeve 202 by the attraction pole S3.
Although the developer G within the image formation range moves as described above, in axial end portions of the development roller 200, the developer G separated from the development sleeve 202 by the upstream release pole S2 tends to move through outer portions where the magnetic force of the magnet roller 203 is not present toward the attraction pole (downstream release pole) S3. This is a phenomenon called “carry-over of developer”.
If the portion where carry-over of developer occurs contribute to image development, the image density of the resultant image decreases in the direction in which the sheet is transported because the developer is not replenished with toner from the developer container.
In this known development device, carry-over of developer is deemed inevitable, and, as shown in FIG. 34B, the development roller 200 is expanded in the longitudinal direction so that carry-over of developer occurs in outer portions Al outside the image formation range. Thus, the development device increases in size by the corresponding amount. As another approach, the shaft of the development roller may be made of a nonmagnetic material. However, the cost is increased when stainless steel is used, and strength is not sufficient when aluminum is used.